Cryosurgery, or the destruction of undesired biological tissues by freezing, has long been accepted as an important alternative technique of surgery (Orpwood, 1981; Rubinsky and Onik, 1991; Gage, 1992). Compared with conventional means of destroying tissues, such as surgical excision, radiotherapy and immunotherapy, visceral cryosurgery (especially minimally invasive cryosurgery) offers the following potential advantages: simplicity of the procedure, minimal bleeding, anaesthetic effect of low temperatures, short period of patient recovery, low cost, minimal scarring, and possible stimulation of the body's immune system.
James Arnott, an English physician, was the first to introduce the technique of destruction of biological tissues by freezing in 1865. Since Arnott's first report, numerous cryodevices and techniques have been suggested. These have included pre-cooled metal blocks, spray/pour freezing with liquefied gases, refrigeration systems, thermoelectric methods, dry ice applications, cryogenic heat pipes, Joule-Thompson effect based cryoprobes and boiling effect based cryoprobes. However, as a result of the high cooling power usually needed for cryosurgery, and especially of internal organs, the boiling effect and the Joule-Thompson effect have been found to be the preferable cooling technique by most cryosurgeons.
Cryosurgical success, or maximal destruction of undesired biological tissues by freezing, is influenced by many factors: the cooling rate (Smith and Fraser, 1974; Gage, 1985; Fahy, 1990), the thawing rate (Miller and Mazur, 1976), the minimal temperature achieved (Gage, 1982), and the number of repeated freezing/thawing cycles (Rand et al., 1985). Many controlled cryodevices and cryoprotocols have been suggested to improve the cryodestruction, where the controlled variable is the cryoprobe temperature. Additional thermocouples, which are distributed in the cryotreated region, are used in some cases as an external feedback for the control system. In some other cases the electrical impedance of the cryotreated tissue is used as an additional indicator of cryodestruction. Most suggested cryoprotocols deal with the cooling stage of the cryoprocedure and therefore extensive efforts have been made to develop cryoprobes that can be accurately controlled within this stage. Some cryoprobes have a further ability of controlled heating at the thawing stage of the cryoprocedure (Fillipi, 1971; Merry and Smidebush, 1990; Rabin et al., 1995). The controlled thawing process is performed either by forcing hot working fluids through the cryoprobe's passageways or by activation of an electrical heater which is an integral part of the cryoprobe.
Cryosurgery of internal organs, and especially minimally invasive cryosurgery, is monitored by one of the following imaging techniques: ultrasound, CT or NMR. However, ultrasound is the most accepted imaging technique among cryosurgeons. Utilizing these techniques, the cryosurgeon inserts the cryoprobe(s) into the expected cryotreated region. Then, the cryosurgeon activates the cryoprobe according to some cooling protocol and monitors the frozen region growth (which is also termed "ice-ball"). When the undesired tissues are completely frozen, or when there is a danger of cryodestruction to important surrounding tissues, the cryosurgeon terminates the cooling process and the thawing stage follows. In some cases the cooling.backslash.thawing stages are repeated in order to increase the cryodestruction.